1. Field of the Invention
The present invention relates to a corona charger used in electrophotographic apparatuses, such as copy machines, printers, and facsimiles, and to an electrophotographic apparatus including the corona charger.
2. Description of the Related Art
A so-called electrophotographic apparatus, which is an image forming apparatus using electrophotography, forms an electrostatic latent image on a surface of an image bearing member (electrophotographic photosensitive member) by charging the surface of the image bearing member and subsequently exposing the image bearing member. In order to charge the surface of the image bearing member, a corona charger, which is a non-contact charging member, is widely used.
A corona charger generally includes a shielding case (supporting enclosure) being open at one side opposing the image bearing member, supporting blocks (holding members) disposed at both ends of the shielding case in the longitudinal direction, and a charging wire (charging line) extending between the supporting blocks. The charging wire is generally made of tungsten or the like.
For charging a surface of an image bearing member with a corona charger, a discharge current is applied to the charging wire with the open side of the shielding case closely opposing the image bearing member, thus generating corona discharge to apply electric charges to the surface of the image bearing member.
The charged potential of the image bearing member is often controlled by adjusting the amount of electric charge applied to the image bearing member by applying a bias voltage a grid disposed between the charging wire and the image bearing member.
The grid may be made of wires of the same material as the charging wire or stainless steel (SUS), or a plate of such a material having a plurality of through-holes formed by etching.
The wire grid is liable to be contaminated with toner or the like. A contaminated grid tends not to sufficiently control the charged potential at the surface of the image bearing member, and the charged potential becomes nonuniform.
On the other hand, the plate grid has a larger area than the wire grid, and can accordingly more easily control the charged potential at the surface of the image bearing member to an appropriate range. In addition, even if the plate grid is contaminated to some extent, the ability to control the charged potential is not degraded more than the wire grid.
Furthermore, a plate grid (base) made of SUS can have high durability, and is accordingly difficult to deform even by long-term use. The ability of the grid to control the charged potential is not changed by the deformation.
Thus, it is expected that the plate grid will control the charged potential at the surface of the image bearing member to a substantially constant value over a long term.
If a corona charger including a plate grid is used under high-temperature, high-humidity conditions, however, nonuniform charging occurs at the surface of the image bearing member in the longitudinal direction of the corona charger. Consequently, a defect, such as unevenness in density, occurs in output images.
This is because the material of the plate grid reacts with a product of corona discharge to produce an insulating metal oxide on the surface of the grid. The insulating metal oxide makes the plate grid partially insulative to reduce the amount of electric charge to the plate grid and increase the amount of electric charge to the image bearing member. Consequently, the surface of the image bearing member has nonuniform charged potentials.
In addition, the product of the corona discharge may clog the through-holes, thereby causing nonuniform charging.
In general, SUS has higher corrosion resistance than other metals in normal atmosphere. SUS has a chromium oxide-based passive layer at the surface. This passive layer isolates the base metal (SUS base) from the external environment. Thus, SUS exhibits high corrosion resistance. Even if the passive layer is destroyed by an oxidizing substance, such as chlorine ion, SUS is self-repaired by Cr or Mo in the SUS. Accordingly, SUS is broadly used as a corrosion-resistant material.
Even though a SUS member is used in the corona charger of an electrophotographic apparatus, however, the above problem can occur.
This is probably because corrosive substances, such as ozone and NOx, are present around the corona charger in higher concentrations than in normal atmosphere.
When SUS is used under high-humidity conditions in which corrosive substances are present in high concentrations, rust or other corrosion is liable to occur faster than the self-repair of the SUS. Cr and other metal atoms in the passive layer destroyed by oxidizing substances, such as ozone and NOx, react with the oxidizing substances to produce discharge products before repairing the passive layer.
In electrophotographic apparatuses, extremely larger amount of oxidizing substances is ionized than in normal atmosphere. In addition, in a high-humidity atmosphere, ozone is dissolved in water in the air to produce free radicals (OH). The free radicals rapidly react with organic and inorganic substances to oxidize the Cr and other metal atoms in the SUS (indirect oxidation by ozone). Consequently, current is likely to flow locally into the portion not coated with the passive layer, where the base metal (SUS base) is not protected, and thus corrosion can easily occur.
In order to solve this problem, Japanese Patent Laid-Open Nos. 11-40316, 2006-113531, and 2007-256397 disclose that the base metal of the plate grid is plated with a rust-preventing material, such as gold.
Even if the base metal of the plate grid is plated, however, the environment in electrophotographic apparatuses may cause the metal of the plate grid to corrode from a pin hole in the plating. While pin holes can be reduced by increasing the purity of the plating, or increasing the thickness of the plating, it is impossible to completely eliminate pin holes. Thus, the corrosion of the plate grid may not be prevented in some cases.
If a pin hole is formed in the plating, air in normal living environment containing oxygen, nitrogen, sulfur, chlorine, nitrogen oxides, carbon dioxide, sodium chloride, and so forth penetrates through the pin hole to produce a local cell, thus corroding the base metal of the plate grid. Also, a potential difference occurs between the plating metal and the base metal to produce a local cell, thus corroding the plate grid. Furthermore, plating considerably increases the cost depending on the plating metal.